1. Technical Field
The invention relates to the field of measuring the relative amount of the two isotopes of uranium hexafluoride. Specifically, the invention is an improved apparatus and method to quickly and accurately measure the ratio of the two isotopes of uranium hexafluoride in the gaseous phase using tunable diode laser spectroscopy (TDLS).
2. Description of Related Art
Uranium is found naturally as two isotopes: 235U and 238U. Uranium is frequently used in the atomic energy sector. Naturally occurring uranium has a 238U composition of 0.7%. Depleted uranium has a 238U composition of about 0.2%. To be used in the atomic energy industry, uranium needs to be enriched to about 5.0% of 238U. Uranium for nuclear ammunitions needs to be enriched more than 20%.
The molecule uranium hexafluoride, UF6, is the single known gaseous uranium molecule used in the field of nuclear energy and has been of significant interest over the last five decades in the separation of uranium isotopes by laser, the separation of 235U from its only other stable isotope 238U and from other chemical species. Isolating 235U from 238U, and from other chemical species, is known as enrichment. Monitoring the relative amounts of 238U and 235U is important during the enrichment process and during periodic verification of stored uranium hexafluoride. For example, as of the year 2000, over 700 million kilograms of depleted uranium hexafluoride (DUF6), containing 475 million kilograms of uranium, have been generated by the U.S. government. Other sources of UF6 exist.
The main advantage of optical, non-destructive methods of detection, such as TDLS, is the possibility of non-contact isotopic measurement combined with real-time data processing. Another benefit is its high selectivity as different molecules and isotopes have different spectra. These spectra vary with temperature and physical phase.
Currently, there are two types of methods to measure individual gas samples: destructive and non-destructive. The destructive method involves collecting samples in special gas containers and processing these samples through a mass spectrometer. The accuracy of this method is about 1% relative. This method is relatively expensive. A mass spectrometer costs on the order of one million U.S. dollars. Each sample is relatively expensive to gather and process. This method is also very slow: each sample can take up to one hour to process.
There are two non-destructive methods to measure uranium hexafluoride. One method uses 241Am as a source of 60 keV gamma-radiation and a low resolution NaI(TI) detector for the measurement of 235U and 238U by detecting the 60 keV and 186 keV gamma radiation, the last is specific only for 235U. The content of 235U is calculated from the ratio of the two measurements. The other method for the measurement of 235U at low pressure uses 57Co as a source of 122 keV gamma-radiation.
Both of these methods have a low accuracy, about 20%, for example, 7%±20%. In the industry, these methods have been used to determine if samples of uranium are either greater than or less than 20% enriched with 235U. However, these methods are useless to distinguish between naturally occurring uranium, having a 0.7% composition of 235U, and depleted uranium, having a 0.2% composition of 235U. In these methods, special care is required to differentiate the measurement of the uranium in the gaseous phase from that of the uranium that deposits on the walls of the sample container.
Mid infrared diode laser spectroscopy (IR-DLS) could be applied to the measurement of the uranium isotope concentration in low pressure gaseous UF6. Uranium hexafluoride has many absorption bands located in the mid and far infrared regions of its vibrational energy spectrum. Specifically, the UF6 molecule has six normal vibrational modes: ν1=667 cm−1, ν2=534 cm−1, ν3=626 cm−1, ν4=186 cm−1, ν5=200 cm−1, and ν6=143 cm−1. For TDLS, the most suitable absorption band is the combination ro-vibration band ν1+ν3 near 7.8 μm because it has a relatively strong absorption in this region of the spectrum. This band has an unresolved PQR structure corresponding to changes in angular momentum J during transition (P: ΔJ=−1, R: ΔJ=+1, Q: ΔJ=0). Normally, it is impossible to resolve individual ro-vibration lines because of their high density and only their broad PQR structure can be observed. The position of the Q-branch in the center of the band is shifted for different uranium isotopes. This shift can easily be detected by using high-resolution TDLS thus making it possible to measure the concentration of each isotope in the gas. A gas sample with an optical path that is 10 cm long is sufficient to detect a reliable measurement.
It would be ideal to have a device and method to quickly and accurately detect the relative amounts of 235U and 238U in a gaseous sample. Further, it would be beneficial to have a device that is capable of recording and analyzing such measurements automatically. Further, it would be beneficial to have a device that required only reference gases instead of a sample of known 235U activity as a calibration for measurement. Further, it would be ideal to have a device that is relatively inexpensive and portable to make measurements. Finally, it would be ideal to have a device that could take advantage of the recently discovered benefits associated with TDLS.